Si Dispersoids Research Articles

Evolution of mechanical properties and microstructure of selective laser melted AlSi10MgMn alloy with different post heat treatments

This study investigated the effects of various post heat treatments on the mechanical properties and microstructure evolution of an AlSi10MgMn alloy containing 0.5 wt% Mn produced by the selective laser melting process for the first time. The microstructures under different conditions were analyzed using optical microscopy, scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. In the as-manufactured (F) condition, the alloy exhibited an ultimate tensile strength (UTS) of 486 MPa, a yield strength (YS) of 299 MPa, and an elongation of 10.3 %. After a T5 treatment, the UTS and YS increased to 532 MPa and 386 MPa, respectively, resulting in a remarkable 30 % improvement in YS compared to the F state. The tensile properties achieved by the new alloy were considerably higher than those reported for conventional AlSi10Mg alloys in the F, T5, and T6 conditions. The T5 treatment promoted the precipitation of a large fraction of Si-rich nanoparticles and MgSi-based precipitates without disrupting the Si-rich network. After a T6 treatment, the Si-rich network completely disappeared, and the main strengthening phase was MgSi-based precipitates accompanied by α-Al(Mn,Fe)Si dispersoids induced by the Mn addition. Using microstructure-based constitutive models, the strengthening contributions of various microstructural components to mechanical strength in different processing conditions were analyzed.

Microstructure evolution of a novel low-silicon cast aluminum alloy modified with Mn and Cr during solution treatment

The evolution of eutectic Si, the precipitation behavior of dispersoids and microhardness change of the α-Al matrix in a Mn + Cr modified Al-Si-Mg-Cu alloy during solution treatment were investigated. The results revealed that the evolution of eutectic Si was divided into four paths during solution treatment: Dissolution, splitting, spheroidization and coarsening. Meanwhile, numerous α-Al(FeMnCr)Si dispersoids were precipitated in the eutectic region and the α-Al dendrite arms after solution treatment for 30 min. Due to differences in precipitation behavior, three different dispersoid precipitation zones were formed in the dendrite arms, including the dispersoid-free zone (DFZ) near the grain boundaries, coarse dispersoid zones (CDZ) located in the dendrite cores, and the remaining fine dispersoid zones (FDZ). The bimodal size distribution of the dispersoids within intradendritic regions were closely related to the micro-segregation of Mn and Cr. The precipitation of coarse grain boundary phases exhausted the Fe solute near the grain boundary, which was mainly responsible for the formation of the DFZ. Moreover, there was a partial coherent interface between the dispersoids and the α-Al matrix. The evolution mechanism of matrix microhardness can be well explained by dispersion strengthening effect

A systematic through-process rolling and extrusion study of four experimental high-strength Al-Mg-Si alloys

Al-Mg-Si alloys are an important class of Al wrought alloys. Further increasing their strength has significant economic potential. Previous approaches were focusing on increasing the amount of the main hardening elements Mg and Si, adding Cu and Zn for additional hardening potential, and adding dispersoid-forming elements such as Cr and Zr. In terms of processing, multi-step heat treatments and interrupted quenching have been pursued. Despite recent advances in the field, data points from the literature are difficult to compare due to widely varying alloy compositions and processing conditions. Here we present a comprehensive through-process rolling and extrusion study on four experimental high-strength Al-Mg-Si alloys (named A1–A4) with a particular focus on comparability of the results between our alloys and with the literature. Mechanical testing after artificial aging and in-depth microstructural investigations at various steps of the processing chain were employed, including scanning and transmission electron microscopy. We found that Cr addition leads to a strong increase in dispersoids and thus has a strong influence on hot workability, whereas the effect of increasing Mg and Si levels was negligible within the studied composition range. Al3Zr dispersoids were found to coexist with Al(Fe,Mn,Cr)Si dispersoids. Overall, the strength of the rolled sheets was slightly higher than that of the profiles. After 8 h of artificial aging at 180 °C, 2 mm thick sheet made of alloy A4, falling within the specification of AA6086, showed a notable yield strength of 393 MPa, due to nanoscale hardening phases that were smaller and appeared more numerous than in a commercial alloy (AA6082-T6).

Enhancing the recrystallization resistance and strength-ductility trade-off of Al–Mg–Si–Cu–Mn alloys by three-step homogenization

This study explores a three-step homogenization regime tailored to optimize the dispersoids precipitation, recrystallization resistance, and consequently, the mechanical performance of an Al–Mg–Si–Cu–Mn alloy. Comparative analyses against conventional one-step homogenization treatment reveal a substantial improvement in the precipitation of α-Al(Mn,Fe)Si dispersoids, with a 77.2% increase in number density and a 9.1% reduction in average size. This enhancement is attributed to the optimized nucleation kinetics achieved at an intermediate temperature during the three-step homogenization process. Samples subjected to one-step homogenization treatment experience complete recrystallization after a 1 h solution treatment at 560 °C, whereas three-step homogenized samples exhibit superior resistance to recrystallization, maintaining a well-preserved recovery structure. Pinning force analysis highlights the enhanced effectiveness of dispersoids obtained with the three-step homogenization treatment in inhibiting recrystallization. In terms of mechanical performance, samples underwent the three-step homogenization treatment demonstrate a synergistic enhancement in strength and ductility compared to the one-step homogenized sample. This enhancement is attributed to the intergranular fracture and low working hardening rate induced by the large recrystallized grains present in the samples treated with one-step homogenization.

Tailoring microstructure and mechanical properties of Al-Mg-Si-Cu alloy with varying Mn and/or Cr additions

Introducing and maintaining the subgrain structure during thermomechanical processes is an effective approach to mitigate the strength-ductility contradiction of Al-Mg-Si-Cu alloys, which holds significant importance for their engineering application. The objective of this work is to explore the effects of individual and synergistic additions of two dispersoid-forming elements, Mn and Cr, on microstructure and associated mechanical properties of an Al-Mg-Si-Cu alloy. The results show that when compared to alloys with individual addition of either Mn or Cr, their synergistic incorporation leads to an improved dispersoid precipitation with reduced size. For instance, the 0.7Mn0.4Cr alloy exhibits a number density of 10.26 μm-2, which is twice that of the sum of 0.7Mn and 0.4Cr alloys. In response to that, the 0.7Mn0.4Cr alloy attains enhanced resistance to recrystallization, leading to a well-maintained recovery structure and exceptional mechanical properties. But an excess addition of Cr will result in the formation of primary Al45Cr7 phase, which significantly deteriorate the mechanical properties of the alloy. While notably, the 0.4Cr alloy surprisingly exhibits better recrystallization resistance compared to the 0.7Mn alloy, despite having larger dispersoid size and lower number density. This can be attributed to the coherent interface between the α-Al(Cr,Fe)Si dispersoids and matrix, providing higher pinning force that effectively impede the recrystallization.

Effect of Cu on the Precipitation of α-Al(Mn,Cr)Si Dispersoids in an Al-Mg-Si-Mn-Cr Alloy

Nanoscale dispersoids will retard or inhibit recrystallization of aluminum alloys during thermomechanical processes. In the present study, the influence of an addition of 0.6 wt. % Cu on the precipitation behavior of dispersoids in an Al-Mg-Si-Mn-Cr alloy had been investigated. Large amounts of dispersoids with different shapes, e.g. cubic, rod-like and plate-like, were achieved in the experimental alloys after homogenization. Compared with the Cu-free alloy, Cu-added alloy exhibits a higher proportion of cubic shape dispersoid. HRTEM results indicated that the cubic shape dispersoid has an icosahedral quasicrystal structure, while the rod-like or plate-like shape dispersoids show a simple cubic crystal structure. Due to the presence of a high number density of quasicrystalline dispersoids, the Cu-added alloy exhibits a higher recrystallization resistance during hot compression. This study presents a new insight that besides the precipitation strengthening, the Cu alloying in an Al-Mg-Si-Mn-Cr alloy can also contributes to the precipitation of dispersoids.

The Effect of Soaking Time on Mechanical Properties of Roll-Bonded AA3003 and AA4045 Used for Heat Exchangers

Due to the rising need for energy saving, high-performing automotive heat exchangers, demand has significantly grown in recent years. As a result, effective fin-tube heat exchangers are becoming more popular. These tubes are typically made by rolling flat strips of AA3003 aluminum alloys that have either one or both sides coated with AA4xxx alloys. The AA3003 type of alloy is typically used as the core, which is then covered in either AA4045 or AA4343, which melts during the brazing process to adhere the fins to the tubes. To maintain the optimal size and distribution of manganese (Mn)-containing precipitates, preheating parameters are carefully controlled. Then, longer soaking times or higher soaking temperatures result in larger precipitates, which cause the final product to exhibit poor mechanical properties. Therefore, it is crucial to optimize the different manufacturing steps, such as homogenization, soaking time, and brazing in order to achieve a high quality product. Studies on the impact of homogenization temperature and time on the microstructure of AA3xxx aluminum alloys have been conducted. However, there has been little research on the impact of soaking (reheating) time on AA3003 cladded alloys. Hence, the effects of isothermal soaking time on the microstructure and mechanical properties of AA3003 cladded with AA4045 alloy were investigated in this work. Optical microscopy (OM) and scanning electron microscopy (SEM) were used to characterize the microstructure and identify intermetallic phases. The final microstructure in terms of grain structure at various homogenization times was characterized by electron backscattered diffraction (EBSD). After the hot-rolling and cold-rolling of the as-received material, large particles of intermetallic (mainly in the form of Chinese script morphology consisting of Fe-Mn-Si) were broken into smaller particles with an increased Fe, Mn, and Si content, indicating the formation of an α-Al(Fe,Mn)Si phase. The α-Al(Mn,Fe)Si was found to be a dominant dispersoid precipitate in the modified AA3003 core. Coarsening of the Al(Mn,Fe)Si dispersoids at 505 °C was only observed at a 45 h homogenization time. The hardness trend with homogenization time was found to be similar to that after homogenization, cold working, and annealing, with exception of an increase in hardness in the latter possibly due to strain hardening (from cold-rolling).

Enhanced long-term thermal stability and mechanical properties of twin-roll cast Al–Mg–Si alloys with Mn and Cu additions

The long-term thermal stability of Al–Mg–Si alloy is of great significance for its engineering application. In this work, the effects of Mn and Cu on the microstructure and mechanical properties of the twin-roll cast (TRC) AA6005 alloy after peak aging and long-term thermal exposure were studied. The results show that the addition of Mn introduces Al(FeMn)Si dispersoids, which improves the mechanical properties of the alloy under the peak aging condition by dispersion strengthening. After thermal exposure at 150 °C for 1000 h, the Al(FeMn)Si dispersoids have good thermal stability while do not significantly improve the mechanical properties. The reason may be related to the decreased number density of the β″ phase after alloying with Mn. The addition of Cu changes the aging precipitation sequence, and the needle-like pre-Q′ phase is formed during artificial aging. Moreover, due to the low diffusion rate of Cu in the Al matrix, Cu atoms can better stabilize the structure and then improve the coarsening resistance of the precipitates, and therefore the Cu-containing precipitates exhibit better thermal stability than the β″ phase. After thermal exposure at 150 °C for 1000 h, the yield strength of the 0.5Cu alloy decreases by only ∼12 MPa (from ∼295 to ∼283 MPa), which is significantly lower than that of the AA6005 alloy (∼41 MPa, from ∼274 to ∼233 MPa). The yield strength of the 0.25Mn0.25Cu alloy decreases by ∼27 MPa (from ∼294 to ∼267 MPa) and is more suitable for industrial production as its composition is within the range of commercial 6xxx aluminum alloys. The results would provide a strategy for the preparation of Al–Mg–Si alloys with high thermal stability.

Effect of Mn/Cr ratio on precipitation behaviors of α-Al(FeMnCr)Si dispersoids and mechanical properties of Al–Mg–Si–Cu alloys

The submicron α-Al(FeMnCr)Si dispersoids in 6XXX aluminum alloys can retain subgrains in the deformed microstructure and improve the mechanical properties of the alloys at elevated temperature. In this study, the precipitation behavior of dispersoids in 6XXX alloys containing various Mn/Cr ratios (0.5–8) during different homogenization heat treatments (soaking at 500, 530, and 560 °C for 0–24 h) was investigated. The effects of the Mn/Cr ratios on the elevated-temperature mechanical properties, dynamic and static recrystallization behaviors of the alloys were evaluated. A suitable Mn/Cr ratio (3.5) contributes to the precipitation of a large number of fine dispersoids, enhancing the dispersion strengthening and Zener drag PZ. Excess Cr incorporation decreased the nucleation rate of the dispersoids which could not be precipitated completely during the homogenization heat treatment, thereby weakening the pinning effect and decreasing the dispersion strengthening effect. During hot deformation and subsequent solution treatment, the M7 alloy achieved the highest percentage of substructures. The M7 alloy exhibited excellent mechanical properties at room and elevated temperatures, and good thermal stability during long-term annealing (300 °C for 100 h).

Effect of floating grains on the deformed microstructure of Mn- and Cr-containing Al-Mg-Si-Cu alloys

The effects of floating grains on the deformed microstructure of a Mn- and Cr-containing Al-Mg-Si-Cu alloy were first clarified based on a comparative study of the dispersoid precipitation and recrystallization behavior with and without floating grains. The presence of floating grains in the direct-chill casting billets resulted in inhomogeneous and sparse precipitation of the α-Al(Mn,Cr)Si dispersoids after homogenization. In response, the Zener drag force was significantly reduced and severe localized recrystallization occurred in the specimen containing floating grains after hot working. Whereas, the specimen without floating grains, which had dense and fine dispersoids, still had an ideal recovery microstructure after solution soaking. The innate reason consists in the special microscale features of floating grains, which significantly decreased the nucleation rate of the dispersoids-forming β′ precipitates.

Strain localization at longitudinal weld seams during plastic deformation of Al–Mg–Si–Mn–Cr extrusions: The role of microstructure

The current work was motivated by the increasing use of hollow aluminum extruded profiles for automotive applications and the need for a better understanding of the mechanical response of extrusion seam welds. Two Al–Mg–Si alloys were studied, one without Mn and one with 0.5 wt%Mn and 0.15 wt%Cr. A high temperature homogenization was applied, resulting in a high density of α-Al(Fe,Mn)Si dispersoids in the Mn containing alloy, and an essentially dispersoid free microstructure in the other. The alloys were extruded through a simple porthole die (with two ports) to produce a strip with a centerline weld seam. The Mn free extrudate had a recrystallized grain structure whereas the Mn and Cr containing material was unrecrystallised. There were significant spatial variations of microstructure between the weld seam and surrounding regions, in particular the crystallographic texture. Tensile tests were conducted with the loading direction perpendicular to the weld seam using digital image correlation (DIC) to characterize the local strain distribution. Strain localization and final fracture occurred at weld seam for the unrecrystallized material whereas these occurred away from the weld line in the recrystallized case. The role of local crystallographic texture was examined through polycrystal plasticity simulations using the visco-plastic self consistent code (VPSC). These simulations indicated that the weld seam region was mechanically stronger than the surrounding region for the recrystallized alloy and correspondingly weaker for the unrecrystallized case, thereby rationalizing the experimental results of strain localization and final fracture location.

Quantitative analysis of influence of α-Al(MnFeCr)Si dispersoids on hot deformation and microstructural evolution of Al-Mg-Si alloys

The microstructural evolution of AA6061 and Mn-bearing Al-Mg-Si-Cu alloys was studied by compression tests that were carried out between 300 and 500°C with a wide range of strain rates. Compared to the AA6061 alloy, the large amount of α-Al(MnFeCr)Si dispersoids in the Mn-bearing alloy yielded a significant increase in the flow stress under all deformation conditions. The effects of the deformation parameters on the evolution of the microstructure were studied using electronic backscatter diffraction measurements. The predominant softening mechanism of both alloys was dynamic recovery. The presence of α dispersoids in Mn-bearing alloys effectively refined the size of substructures with misorientation angles in the range of 2°-5°, which retarded the dynamic recovery. To predict the subgrain size under various deformation conditions, the threshold stresses that were caused by α dispersoids were calculated by the modified Orowan equation and incorporated into a conventional constitutive equation. The subgrain size that was predicted by the modified constitutive equation showed satisfactory agreement with the experimental measurements.

Precipitation Characteristics of HPVDC AlSi10Mg0.3Mn Alloy Under Different Temper Conditions

This work studied the precipitation characteristics of AlSi10Mg0.3Mn alloy produced by high-pressure vacuum die-casting in T5 and T6 conditions using transmission electron microscopy, differential scanning calorimetry and both electrical conductivity and microhardness measurements. Two different T6 tempers were used, involving partial solutionizing at 460 °C for 1 h and full solutionizing at 500 °C for 1 h, and were designated as T6P and T6F, respectively. The aging treatment was conducted at 185 °C for 4 h in all temper conditions. The results showed that conducting solution treatments resulted in the formation of α-Al(MnFe)Si dispersoids. In addition, Mg–Si rich precipitates formed and remained undissolved after partial solutionizing. The highest supersaturation of the α-Al was achieved in the as-fabricated (F) condition by the HPVDC process. The supersaturation degree decreased in both partial and full solutionizing conditions. TEM observations revealed different precipitation characteristics for T5, T6P and T6F tempers. β” precipitates just started to form in the T5 condition, whereas both β” and β’ precipitates completed their precipitation to different extents in the T6P and T6F conditions, resulting in different strengthening effects. The T6F temper yielded the highest microhardness followed by the T5 and T6P tempers.

Application of Color Metallography on As-cast Al-Mg-Si-Cu-Mn Alloy during Heat Treatment

Color etching was performed to characterize microstructure evolution and phase transformation on as-cast and homogenized Al-Mg-Si-Cu-Mn alloys. Initial intermetallic phases in microstructure of as-cast Al-Mg-Si-Cu-Mn alloy are mainly composed of primary Mg2Si, α-Al(FeMn)Si and quaternary Q phase. Post-etching microstructure of as-cast alloy presents a typical dendritic morphology with distinctive intermetallic distributions. Micro-segregation of solute elements are visualized as color difference within grains. Q phase is mainly located between the dendrite arms, while Mg2Si and α-Al(FeMn)Si are arranged along the grain boundaries. Heterogeneous nucleation sites provided by Al3Ti during solidification are also observed and identified at the core of grains. The distribution of alloying elements and intermetallic exhibited by color micrograph presents a good agreement with outcomes of EPMA. The color metallography of etched sample reveals the distribution of α-Al(MnCr)Si dispersoids and dispersoids free zone (DFZ), after the alloy is subjected to two-step homogenization. Micro-segregation of solute elements within solid solution is dramatically eliminated, associated with reduction of color difference. The dissolution of primary Mg2Si and Q phase during two-step homogenization are also directly detected. Therefore, color etching is an effective and reliable auxiliary approach to reveal microstructural evolution and phase transformation of as-cast Al-Mg-Si-Cu-Mn alloy during homogenization process.

Microstructure and elevated-temperature mechanical properties of dispersoid-strengthened Al-Mg-Si-Mn alloys produced by twin roll casting

The microstructure and elevated-temperature mechanical properties of dispersoid-strengthened Al-Mg-Si-Mn alloys produced by twin roll casting (TRC) and conventional mold casting (MC) were studied. A specific heat treatment at 430 ​°C for 6 ​h was applied, followed by rolling. Tensile tests were carried out at 300 ​°C. The results show that in the as-cast microstructures, equiaxed grains formed in the MC sample while elongated grains with substructures formed in the TRC sample. The eutectics in the TRC sample were much finer than those in the MC sample. Upon heat treatment, large numbers of α-Al(Fe, Mn)Si dispersoids precipitated, and the TRC sample had a greater number density of dispersoids than those in the MC sample. During the tensile test after rolling, the TRC samples exhibited higher strength than the MC samples, which resulted from the combination of the strengthening effect of the stronger dispersoids and the greater substructure hardening effect in the TRC sample; the TRC sample exhibited better ductility due to its finer initial eutectic than that in the MC sample, which depressed the stress concentration and delayed the occurrence of microcracks and fracture in the TRC sample.

Effects of Zr and Sc additions on precipitation of α-Al(FeMn)Si dispersoids under various heat treatments in Al–Mg–Si AA6082 alloys

Abstract The present work investigated the influence of Zr and Sc on the evolution of α-Al(FeMn)Si dispersoids (“α-dispersoids") in Al–Mg–Si alloys. Both the individual addition of Zr and the combined additions of Sc and Zr increased the size but decreased the number density of the α-dispersoids, indicating the reduction in the formation of α-dispersoids. However, the reduction levels were the most significant when heat-treated at 350 °C in the alloy with both Sc and Zr and at 400 °C in the alloy with only Zr, which were likely related to the different interactions between intermediate B’ precipitates and α-dispersoids with the addition of Zr and Sc. Although the α-dispersoids were suppressed in the Zr/Sc-containing alloys, their microhardness was generally higher than the base alloy, which can be attributed to the strengthening contribution induced by Zr and Sc either from their solid solution hardening or the precipitation hardening of Al3Zr/Al3(Sc, Zr) dispersoids.

Enhanced Elevated-Temperature Strength and Creep Resistance of Dispersion-Strengthened Al-Mg-Si-Mn AA6082 Alloys through Modified Processing Route.

In the present work, we investigated the possibility of introducing fine and densely distributed α-Al(MnFe)Si dispersoids into the microstructure of extruded Al-Mg-Si-Mn AA6082 alloys containing 0.5 and 1 wt % Mn through tailoring the processing route as well as their effects on room- and elevated-temperature strength and creep resistance. The results show that the fine dispersoids formed during low-temperature homogenization experienced less coarsening when subsequently extruded at 350 °C than when subjected to a more typical high-temperature extrusion at 500 °C. After aging, a significant strengthening effect was produced by β″ precipitates in all conditions studied. Fine dispersoids offered complimentary strengthening, further enhancing the room-temperature compressive yield strength by up to 72–77 MPa (≈28%) relative to the alloy with coarse dispersoids. During thermal exposure at 300 °C for 100 h, β″ precipitates transformed into undesirable β-Mg2Si, while thermally stable dispersoids provided the predominant elevated-temperature strengthening effect. Compared to the base case with coarse dispersoids, fine and densely distributed dispersoids with the new processing route more than doubled the yield strength at 300 °C. In addition, finer dispersoids obtained by extrusion at 350 °C improved the yield strength at 300 °C by 17% compared to that at 500 °C. The creep resistance at 300 °C was greatly improved by an order of magnitude from the coarse dispersoid condition to one containing fine and densely distributed dispersoids, highlighting the high efficacy of the new processing route in enhancing the elevated-temperature properties of extruded Al-Mg-Si-Mn alloys.

Precipitation of dispersoids in Al–Mg–Si alloys with Cu addition

Nanoscale α-Al(FeMn)Si dispersoids in Al alloys play a vital role in improving the recrystallization resistance and mechanical properties of alloys at elevated temperatures. This study investigates the influence of Cu on the precipitation behavior of α-dispersoids in Al–Mg–Si alloys during heat treatment at 500 °C. α-dispersoids in an alloy without Cu addition had rod- or plate-like morphologies. The addition of Cu effectively inhibits the precipitation of α-dispersoids through the formation of fine spherical dispersoids, resulting enhanced mechanical properties of the alloy at elevated temperatures. Evidence from high-resolution transmission electron microscopy demonstrated that the rod- or plate-like structures have simple cubic structures, whereas the spherical α-dispersoids are coherent with the Al matrix along the [101] zone axis of Al, resulting in finer dispersoids, higher thermal stability, and lower coarsening rates. However, its crystal structure requires further investigation.

Improved Distribution and Uniformity of α-Al(Mn,Cr)Si Dispersoids in Al-Mg-Si-Cu-Mn (6xxx) Alloys by Two-Step Homogenization

Improved Distribution and Uniformity of α-Al(Mn,Cr)Si Dispersoids in Al-Mg-Si-Cu-Mn (6xxx) Alloys by Two-Step Homogenization

Analysis on the hot deformation flow stress curves of novel 6082 aluminium alloys with Mn addition

Two types of 6082 alloy, base alloy free of Mn and novel 0.75Mn alloy with an addition of 0.75% Mn, were prepared. A heat treatment of 450 °C for 6 h was performed, during which large numbers of α-Al (Fe, Mn) Si dispersoids precipitated in 0.75Mn alloy. Uniaxial compression tests were carried out and a series of flow stress curves were obtained at temperature range of 400-550 °C and at strain rate range of 0.001s-1-1s-1. The results revealed an impressive difference between the flow stress curves of base and 0.75Mn alloy that after the yield plateau, the flow stresses of base alloy remained fairly steady at certain levels while the flow stress in 0.75Mn alloy decreased along with strain until the end of deformation. The reason for the decrease of flow stress in 0.75Mn alloy was neither the dissolution of dispersoids nor the dynamic recrystallization. The decrease of flow stress was attributed to the loss of Mn solid solution strengthening effect based on the Mn diffusion rate-controlling mechanism and “pipe diffusion”.